The problem with executables is that when they are referenced, theyget a *lot* of references compared to data pages. Think about aninstruction stream and how many of those instructions result in datareferences. So when you see an executable page that is being accessed,it's likely being accessed at a high rate. They're much hotter, andthat's why reference bits from VM_EXEC mappings carry more weight.

IMO this applies to executable file and anon equally.

> > Seems to me you don't need to add anything. Just remove the> > PageSwapBacked branch and apply equal treatment to both types.> > I will rework the code if you agree with my opinion.> > > > @@ -2056,6 +2063,15 @@ static void shrink_active_list(unsigned long nr_to_scan,> > > }> > > }> > >> > > + /*> > > + * Now, newly created anonymous page isn't appened to the> > > + * active list. We don't need to clear the reference bit here.> > > + */> > > + if (PageSwapBacked(page)) {> > > + ClearPageReferenced(page);> > > + goto deactivate;> > > + }> >> > I don't understand this.> >> > If you don't clear the pte references, you're leaving behind stale> > data. You already decide here that we consider the page referenced> > when it reaches the end of the inactive list, regardless of what> > happens in between. That makes the deactivation kind of useless.> > My idea is that the pages newly appended to the inactive list, for example,> a newly allocated anon page or deactivated page, start at the same line.> A newly allocated anon page would have a mapping (reference) so I> made this code to help for deactivated page to have a mapping (reference).> I think that there is no reason to devalue the page accessed on active list.

I don't think that leads to desirable behavior, because it causes anage inversion between deactivated and freshly instantiated pages.

We know the new page was referenced when it entered the head of theinactive list. However, the old page's reference could be much, muchlonger in the past. Hours ago. So when they both reach the end of thelist, we treat them as equally hot even though the new page has beenreferenced very recently and the old page might be completely stale.

Keep in mind that we only deactivate in the first place because theinactive list is struggling and we need to get rid of stale activepages. We're in a workingset transition and *should* be giving oldpages the chance to move out quickly.

> Before this patch is applied, all newly allocated anon page are started> at the active list so clearing the pte reference on deactivation is required> to check the further access. However, it is not the case so I skip it here.> > > And it blurs the lines between the inactive and active list.> >> > shrink_page_list() (and page_check_references()) are written with the> > notion that any references they look at are from the inactive list. If> > you carry over stale data, this can cause more subtle bugs later on.> > It's not. For file page, PageReferenced() is maintained even if deactivation> happens and it means one reference.

shrink_page_list() doesn't honor PageReferenced as a reference.

PG_referenced is primarily for the mark_page_accessed() state machine,which is different from the reclaim scanner's reference tracking: forunmapped pages we can detect accesses in realtime and don't need thereference sampling from LRU cycle to LRU cycle. The bit carries over adeactivation, but it doesn't prevent reclaim from freeing the page.For mapped pages, we sample references using the LRU cycles, andPG_referenced is otherwise unused. We repurpose it to implementsecond-chance tracking of inactive pages with pte refs. It countsinactive list cycles, not references.

> > And again, I don't quite understand why anon would need different> > treatment here than file.> > In order to preserve the current behaviour for the file page, I leave the code> as is for the file page and change the code for the anon page. There is> fundamental difference between them such as how referenced is checked,> accessed by mapping and accessed by syscall. I think that some difference> would be admitted.

Right, unmapped pages have their own reference tracking system becausethey can be detected synchronously.

My questions center around this:

We have an existing sampling algorithm for the coarse-grained pagetable referenced bit, where we start pages on inactive, treatreferences a certain way, target a certain inactive:active ratio, userefault information to detect workingset transitions etc. Anon used adifferent system in the past, but your patch set switches it over tothe more universal model we have developed for mapped file pages.However, you don't switch it over to this exact model we have formapped files, but rather a slightly modified version. And I don'tquite understand the rationale behind the individual modifications.

So let me turn it around. What would be the downsides of aging mappedanon exactly the same way we age mapped files? Can we identify wheredifferences are necessary and document them?